- patient has manifestation that meets case definition of severe covid-19: adult with clinical signs of pneumonia (fever, cough, dyspnea, fast breathing) plus one of the following: respiratory rate > 30 breaths/min; severe respiratory distress; or spo2 < 90% on room air. - fever (37.5°c) more than 7 days after the onset of fever - patients with suspected concomitant bacterial infections (e.g. 1 ng/ml or higher procalcitonin, etc.) prior to initiation of study drug - patients with suspected concomitant fungal infections (e.g. 30 pg/ml or higher (1-3)-β-d-glucan, etc.) prior to initiation of study drug - patients with suspected concurrent congestive heart failure (e.g. 100 pg/ml or higher nt-pro bnp levels, etc.) prior to initiation of study drug - patients with severe hepatic impairment (>grade 3: alt >10 times of upper normal limit) - patients with renal impairment requiring dialysis - patients with disturbed consciousness such as disturbed orientation - pregnant or possibly pregnant patients - female patients who are unable to consent to contraceptive use of oral contraceptives, mechanical contraceptives such as intrauterine devices or barrier devices (pessaries, condoms), or a combination of these devices during the 7 days after the start of favipiravir administration. - male patients whose partner cannot agree to use the contraception method described in (10) above - patients who cannot consent to the use of condoms from the start of favipiravir administration to 7 days after the end of favipiravir administration - patients with hereditary xanthinuria - patients who have previously ever been diagnosed with hypouricemia (< 1 mg/dl) or xanthine urinary calculi - patients with a history of gout or on treatment for gout or hyperuricemia - patients receiving immunosuppressants - patients who received interferon-alpha or drugs with reported antiviral activity against sars-cov-2 (hydroxychloroquine sulfate, chloroquine phosphate, lopinavir-ritonavir combination, ciclesonide, nafamostat mesylate, camostat mesylate) within 9 days after fever (37.5°c or more). - patients in whom this episode of infection is a recurrence or reinfection of sars-cov-2 infection - patients who have previously received favipiravir (t-705a) - other patients judged ineligible by the principal investigator or sub-investigator

- patient has manifestation that meets case definition of severe covid-19: adult with clinical signs of pneumonia (fever, cough, dyspnea, fast breathing) plus one of the following: respiratory rate > 30 breaths/min; severe respiratory distress; or spo2 < 90% on room air. - fever (37.5°c) more than 7 days after the onset of fever - patients with suspected concomitant bacterial infections (e.g. 1 ng/ml or higher procalcitonin, etc.) prior to initiation of study drug - patients with suspected concomitant fungal infections (e.g. 30 pg/ml or higher (1-3)-β-d-glucan, etc.) prior to initiation of study drug - patients with suspected concurrent congestive heart failure (e.g. 100 pg/ml or higher nt-pro bnp levels, etc.) prior to initiation of study drug - patients with severe hepatic impairment (>grade 3: alt >10 times of upper normal limit) - patients with renal impairment requiring dialysis - patients with disturbed consciousness such as disturbed orientation - pregnant or possibly pregnant patients - female patients who are unable to consent to contraceptive use of oral contraceptives, mechanical contraceptives such as intrauterine devices or barrier devices (pessaries, condoms), or a combination of these devices during the 7 days after the start of favipiravir administration. - male patients whose partner cannot agree to use the contraception method described in (10) above - patients who cannot consent to the use of condoms from the start of favipiravir administration to 7 days after the end of favipiravir administration - patients with hereditary xanthinuria - patients who have previously ever been diagnosed with hypouricemia (< 1 mg/dl) or xanthine urinary calculi - patients with a history of gout or on treatment for gout or hyperuricemia - patients receiving immunosuppressants - patients who received interferon-alpha or drugs with reported antiviral activity against sars-cov-2 (hydroxychloroquine sulfate, chloroquine phosphate, lopinavir-ritonavir combination, ciclesonide, nafamostat mesylate, camostat mesylate) within 9 days after fever (37.5°c or more). - patients in whom this episode of infection is a recurrence or reinfection of sars-cov-2 infection - patients who have previously received favipiravir (t-705a) - other patients judged ineligible by the principal investigator or sub-investigator